1. Field of the Invention
The present invention relates to a meter driving apparatus used for automotive meters such as a tachometer and a speedometer.
2. Description of the Prior Art
A general automotive speedometer is provided with a magnet M for swinging an indicator in the magnetic field formed by two orthogonal coils L.sub.s and L.sub.c, as shown in FIG. 3. The current corresponding to an input such as speed is applied to the coils L.sub.s and L.sub.c so as to rotate the magnet M, thereby swinging the indicator. In a meter driving apparatus for a speedometer of this kind, a pulse signal the period of which changes in correspondence with an input, namely, speed is input as well as a clock signal generated by a reference clock generator, the clock signals of a counter are first counted for one period of the pulse signal so as to obtain the period T of the pulse signal, and the frequency f is calculated from f=1/T on the basis of the thus-obtained period T. Since this digital data f is proportional to the input, the current corresponding to the digital value f is applied to the coils L.sub.s and L.sub.c, and the magnetic field generated by the coils L.sub.s and L.sub.c rotates the indicator, which is the magnet, to the direction at an angle determined by the digital value f.
The above-described conventional meter driving apparatus has the following problems. Since the frequency of a pulse signal is obtained by calculating the period of the pulse signal which changes with speed on the basis of the count value of clock signals and the direction is determined by an angle determined by the digital value converted from the frequency by the sin and cos calculations, for example, when the input is rapidly changed from a high speed to a low speed, the frequency of the input pulse signal is also changed from a high frequency to a low frequency and the period becomes long. It takes a long time for the indicator to point in a new direction because the frequency after the change is not calculated and, hence, the indicator does not move until the clock signals for one period are counted. Thus, the responsiveness of the speedometer is very bad.
In addition, since the indicator is swung by the current applied to the orthogonal coils in correspondence with the input by the sin and cos calculations of the frequency, namely, the digital data, for example, when the indicator is rapidly changed from the speed of 145 km/H to the speed of 25 km/H, as shown in FIG. 4, the indicator sometimes swings not in the direction A but in the direction of B in which the values are higher in spite of the actual change from a high speed to a low speed, and overruns the measurable range.